Device for determining the fineness of mineral fibers

ABSTRACT

An apparatus for determining the fineness index of mineral fibers, designed to deliver the micronaire value of fibers. The apparatus includes a device for measuring the fineness index provided with at least a first orifice connected to a measurement cell configured to hold a specimen of a plurality of fiber and provided with a second orifice connected to a device for measuring a differential pressure, on either side of the specimen. The device measuring the differential pressure is configured to be connected to a device for producing a fluid flow. The device for measuring the fineness index also includes at least one flowmeter for measuring the volume flow rate of the fluid passing through the cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is the U.S. counterpart of WO 03/098209 and inturn claims priority of French Application No. 02/06252 filed on May 22,2002, the entire contents of each of which are hereby incorporatedherein by reference.

The present invention relates to an apparatus for determining thefineness of mineral fibres, these fibres being intended especially forthe industrial manufacture of glass wool to be used, for example, inmaking up thermal and/or acoustic insulation products. It also relatesto a method of measuring the fineness of mineral fibres and to theapplication of this apparatus to the measurement of the fineness ofhyperfine fibres.

More precisely the aim is to allow the value of their “micronaire” (F)to be determined.

It is known in this technical field to measure the “micronaire” in orderto characterize the fineness index, this index, accounting for thespecific surface area, being determined by measuring the aerodynamicpressure drop when a given quantity of fibres, containing no binder, issubjected to a given pressure of a gas, generally air or nitrogen.

This measurement is commonplace in natural fibre (especially cottonfibre) production units and is standardized according to the DIN 53941,ASTM 1994, D 4604-86, D 4605-86 or EN 29053 standards and it uses anapparatus called a “micronaire apparatus” as described for example in EP0 373 058.

In addition, it should be noted that the mass of the specimen isgenerally of the order of 10 g and may, according to the recommendationsmost commonly employed, be up to 50 g when the fibres are cotton fibres.

On the basis of these teachings, a micronaire apparatus has beendeveloped, and is widely used, which gives, using a device for measuringthe fineness index of mineral fibres, which is graduated in “micronaire”values, reliable measurements for mean fibre diameters of between about3 and 9 μm and within this mean diameter range. This mean diameter iscalculated from the histogram measured in a microscope (×1000) on 200fibres.

Manufacturers, in their constant search to improve the overall heatexchange coefficient and/or to reduce the density of products for thesame thermal capacity, are increasingly having to produce fibres (calledhyperfine fibres in the rest of the text) whose mean diameter isconstantly being reduced and tends to lie in the range from 2 to 3 μm,or even much lower.

However, in these hyperfine fibre manufacturing ranges, the “micronaire”measurement described above is not possible.

However, such manufacture of hyperfine fibres needs to have ameasurement instrument for monitoring, practically in real time, themanufacturing process so as to be able to rapidly intervene in theprocess should there be a malfunction.

For the sake of rationalization, the assumption adopted was to retainthe principle of the conventional micronaire apparatus, this apparatushaving proven its effectiveness in the case of mineral fibres whosediameter is greater than 3 μm and whose advantages are appreciated byprofessionals in the technical field in question (easy implementation,great simplicity, reliability, speed of implementation, economics, etc).

Based on this observation and knowing the inability of this micronaireapparatus to deliver measurements for hyperfine fibres, the aim of thepresent invention is to provide a micronaire apparatus designed todeliver micronaire values for hyperfine fibres, the mean diameter ofwhich is less than 3 μm.

An apparatus is known for determining the fineness index of fibres whichis designed to deliver the micronaire value of mineral fibres whose meandiameter is substantially greater than 3 μm, comprising a device formeasuring the fineness index, the said device for measuring the finenessindex being provided, on the one hand, with at least a first orificeconnected to a measurement cell designed to hold a specimen consistingof a plurality of fibres and, on the other hand, with a second orificeconnected to a device for measuring a differential pressure, on eitherside of the said specimen, the said device for measuring thedifferential pressure being intended to be connected to a device forproducing a fluid flow.

For this purpose, according to the invention, an apparatus fordetermining the fineness index of the kind in question is characterizedin that the device for measuring the fineness index includes at leastone flowmeter for measuring the volume flow rate of the gas passingthrough the said cell.

By virtue of these arrangements, a micronaire value for hyperfine fibresis rapidly obtained using a conventional apparatus for determining thefineness index without having to modify its calibration and itsoperating conditions (quantity of fibres introduced into the cell, valueof the differential pressure, etc).

In preferred embodiments of the invention, one or both of the followingprovisions may furthermore be optionally adopted:

-   -   the volume flowmeter consists of a volume flowmeter graduated in        l/min;    -   one of the orifices includes a calibrated member.

According to another aspect of the invention, this also relates to amethod of measuring the fineness of fibres using an apparatus fordetermining the fineness of mineral fibres as indicated above,characterized in that:

-   the measurement cell is filled with a specimen of fibres, the    specimen mass being determined in such a way that the specimen    occupies the entire volume of the said measurement cell so that    there is no preferential flow of gas within the specimen;-   the value of the differential pressure between the upstream end and    the downstream end of the specimen is adjusted after the said cell    has been closed using a lid;-   the said measurement apparatus is connected to a device for    producing a flow of fluid; and-   the measurement is taken using the volume flowmeter.

In preferred ways of implementing the invention, one or both of thefollowing provisions may optionally furthermore be adopted:

-   -   the entire volume of the measurement cell is filled with a        specimen of fibres, the mass of which is equal to at least 5 g,        preferably between 5 and 10 g, and even more preferably equal to        5 g;    -   a differential pressure, the value of which is approximately 254        mmHg, is applied.

According to yet another aspect of the invention, this also relates tothe application of the measurement apparatus described above to themeasurement of the fineness index for hyperfine insulation fibres (meandiameter less than 3 μm), especially glass wool, particularly thoseobtained by an internal centrifuging process.

Other features and advantages of the invention will become apparent inthe course of the following description of one of its embodiments, givenby way of non-limiting example and with regard to the appended drawings.

In the drawings:

FIG. 1 is a schematic view of a “micronaire” apparatus according to theinvention; and

FIG. 2 gives the value of the thermal conductivity coefficient forvarious fibre densities as a function of the micronaire value expressedin l/min.

In the various figures, the same references denote identical or similarelements.

FIG. 1 shows a micronaire apparatus according to the invention. Thisapparatus 1 includes a measurement cell 2, which is preferablycylindrical. This cell 2 is designed to hold a specimen of fibres, forwhich it is desired to measure their fineness.

This apparatus also includes a device 8 for measuring the finenessindex.

This device 8 for measuring the fineness index has at least a firstorifice 3 connected via a first pipe 4 to the measurement cell 2designed to hold the specimen formed by a plurality of fibres.

This same device 8 for measuring the fineness index also has a secondorifice 6 connected via a second pipe 7 to a device 5 for measuring adifferential pressure on either side of the said specimen. This device 5for measuring the differential pressure is formed, for example, by apressure regulator whose value is set and is kept constant at 254 mm Hg.

In a conventional “micronaire” apparatus (that is to say one designed toindicate “micronaire” values for fibres whose mean diameter lies withinthe 3 to 9 μm range), the device 8 for measuring the fineness index isformed by a transparent cylindrical tube of circular cross section,within which a movable indicator (of the “ludion” type) can move, thistube being graduated in “micronaire” value.

The device 5 for measuring the pressure differential is connected via athird pipe 9 to a device 10 for producing a constant flow of gas, itbeing possible for the gas to be conventionally air or nitrogen.Whatever the gas flow source used, the installation must allow the flowrate to be accurately controlled and the stability of the gas flow inthe lower part of the measurement cell to be monitored.

The fluid flow source must deliver the fluid at flow rates such that theresulting velocities are low enough for the measured values of theresistance to the fluid flow to be independent of velocity.

To take an example, the flow source must be able to achieve fluid flowvelocities possibly down to 0.5×10⁻³ m/s.

As a variant, the “micronaire” apparatus shown in FIG. 1 includes acalibrated nozzle 12 at the orifice 6 or a calibrated nozzle 11 at theorifice 3. This calibrated member and especially the diameter of theorifices of the nozzle are determined when calibrating the “micronaire”apparatus under standard temperature and pressure conditions (T=20° C.;P=101 325 Pa).

The principle of the fineness index (or micronaire value) measurement isbased on a measurement of the permeability of a porous, homogeneous andisotropic medium, a gaseous fluid flowing under laminar flow conditionsthrough this porous medium (in this case a specimen of fibres whose meandiameter it is desired to determine).

The principle of the measurement is governed by Darcy's law:

$\begin{matrix}{v = {\frac{Q}{S} = {\frac{K}{\mu} \times \frac{\Delta\; P}{e}}}} & (1)\end{matrix}$

-   v: velocity of the fluid (in m/s)-   Q: flow rate of the fluid (in m³/s)-   S: flow area of the specimen through which the fluid flows    perpendicularly (in m²)-   ΔP: differential pressure across the specimen (in N/m²)-   e: thickness of the specimen (in m)-   μ: dynamic viscosity of the fluid (in N/m.s)-   K: permeability (in m²).

Studies carried out by Kozeny and Carman have shown that the coefficientK can be expressed in the following manner:

$\begin{matrix}{K = {\frac{1}{j} \times \frac{ɛ^{3}}{\left( {1 - ɛ} \right)^{2}} \times \frac{1}{\left( S_{v} \right)^{2}}}} & (2)\end{matrix}$where:

-   K: permeability-   j: structure factor

${ɛ\text{:}\mspace{14mu}{porosity}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{medium}} = {1 - \frac{m}{\rho \times S \times e}}$

-   Sv: specific surface area-   M: mass of the specimen-   ρ: density of the specimen.

By combining equations (1) and (2) and by keeping the followingparameters (density, porosity, structure factor, quantity of specimenintroduced) constant, the flow rate Q varies inversely proportionally tothe square root of the specific surface area according to the followingequation:

$Q = \frac{1}{\left( S_{v} \right)^{2}}$

The method of operating the “micronaire” apparatus according to theinvention is as follows.

Using an apparatus for determining the fineness of fibres of the priorart (that is to say one designed to deliver “micronaire” values forfibres whose mean diameter is substantially within the 3 to 9 μm range),and after having calibrated this apparatus with respect to a referenceapparatus using a specimen of reference fibres, so as to obtain thecurve shown in FIG. 2, the measurement cell 2 is filled with at least 5g of fibres whose fineness it is desired to determine (these fibresbeing virgin fibres, that is to say containing no binder).

It should be pointed out the quantity of fibre introduced into themeasurement cell is key. It has been noted that, with a quantity offibre of less than 5 g (for example 3 g), especially when these fibresare hyperfine fibres, the latter do not occupy the entire volume of themeasurement cell and preferential flow regions are created within thecell which singularly affect the measurement. Thus, it has been observedthat the results of the measurement are not reproducible (more than 50%variation in the result).

In practice, tests have been carried out experimentally on variousquantities of fibre and it was determined that the optimum quantity forobtaining satisfactory reproducibility of the measurement consists intaking at least 5 g of fibres.

The measurement cell 2 is filled with 5 g of virgin fibres, the mass offibres being measured using a precision apparatus (the mass of fibres infact lying within the 5.00±0.01 g range), this mass of fibres havingbeen taken using a scoop, just after the fiberizing tool, and before thebinder is deposited.

The measurement cell 2 is a cylindrical chamber whose characteristicdimensions are the following: Inside diameter: 25.4 mm; height: 25.4 mm,the opening of the cell being closed off by a cover 13.

The next step consists in regulating the static differential pressurebetween the upstream end and the downstream end of the specimen offibres; by design, this pressure difference is set to 254 mm Hg.

The apparatus for determining the fineness according to the invention isconnected to the device for producing the gas flow and the measurementis taken using the indicator of the volume flowmeter 20.

Plotted in FIG. 2, for various fibre manufacturing runs, correspondingto a range of three families of well-defined products, each of thesefamilies being characterized by the value of its thermal conductivitycoefficient (λ), is the change in the thermal conductivity for variousmicronaire values, obtained by the apparatus forming the subject of theinvention.

Thus, the following are defined:

-   -   fibres for light wound products (or IBR) having a density M_(v),        between 10 and 11 kg/m³;    -   fibres for dense rolled products or for lightweight panels or        partitions, having a density between 15 and 16 kg/m³;    -   and finally, fibres for dense panels having a density between 22        and 23 kg/m³.

By way of indication, it may be noted that there is a correspondencerelationship between the micronaire values thus obtained and the meandiameter of the fibres of the specimen. In general, a micronaire valueof about 12 l/min corresponds to a mean diameter of 2.5 to 3 μm, a valueof 13.5 l/min correspond to approximately a mean diameter of 3 to 3.5 μmand finally 18 l/min corresponds to about 4 to 5 μm. It will be recalledthat, for these three product ranges, the micronaire value (obtained bya conventional apparatus) cannot be obtained for a mean diameter of 2.5to 3 μm, it is equal to approximately 2.7 for a mean diameter of 3 to3.5 μm, and finally is equal to 3 for about 4 to 5 μm.

Of course, depending on the range of fibres whose mean diameter it isdesired to characterize, it is possible to adapt the device formeasuring the fineness (range of the volume flowmeter) so as to obtainthe same measurement unit (l/min) for all desired fibre production runs.

Thus, for even finer fibres (mean diameter of the order of about 1 μm,or even less), it is possible with the apparatus forming the subject ofthe invention to obtain a reproducible and reliable value, namely of theorder of 1 l/min.

1. An apparatus for determining fineness index of mineral fibers, todeliver a micronaire value of fibers, the apparatus comprising: ameasurement cell configured to hold a specimen of a plurality of fibers;a device for producing a fluid flow; a device for measuring adifferential pressure, on either side of the specimen, the device formeasuring the differential pressure configured to be connected to thedevice for producing a fluid flow; a device configured to measure thefineness index, the device comprising: at least a first orificeconnected to the measurement cell; at least a second orifice connectedto the device for measuring a differential pressure; and at least oneflowmeter configured to measure a volume flow rate of fluid passingthrough the measurement cell.
 2. An apparatus according to claim 1,wherein the flowmeter is graduated in l/mm.
 3. An apparatus according toclaim 1, wherein the second orifice includes a calibrated member.
 4. Anapparatus according to claim 1, wherein the device configured to measurethe fineness index is configured according to a range of fibers whosemean diameter it is desired to characterize.
 5. A method of measuringfineness of mineral fibers, the method comprising: filling a measurementcell with a specimen of fibers such that the specimen occupies an entirevolume of the measurement cell so that there is no preferential flow ofgas within the specimen; adjusting a value of differential pressurebetween an upstream end and a downstream end of the specimen after themeasurement cell has been closed using a lid; connecting the measurementcell to a device for producing a flow of fluid; and taking a measurementof fluid flow using at least one flowmeter.
 6. A method according toclaim 5, wherein the entire volume of the measurement cell is filledwith a specimen of fibers, a mass of the specimen of fibers is equal to5 g.
 7. A method according to claim 5, wherein the mass of the specimenof fibers is between 5 and 10 g.
 8. A method according to claim 5,wherein the value of the differential pressure is approximately 254mmHg.
 9. A method according to claim 5, wherein the mean fiber diameterof the specimen of fibers is less than 3 μm.
 10. A method according toclaim 5, wherein the specimen of fibers are insulation fibers of glasswool.
 11. A method of measuring fineness of mineral fibers, the methodcomprising: providing a measurement cell configured to hold a specimenof a plurality of fibers; providing a device for producing a fluid flow;providing a device for measuring a differential pressure, on either sideof the specimen, the device for measuring the differential pressureconnected to the device for producing a fluid flow; providing a deviceconfigured to measure the fineness index, the device comprising at leasta first orifice connected to the measurement cell; at least a secondorifice connected to the device for measuring a differential pressure;and at least one flowmeter configured to measure a volume flow rate offluid passing through the measurement cell; filling the measurement cellwith a specimen of fibers such that the specimen occupies an entirevolume of the measurement cell so that there is no preferential flow ofgas within the specimen; adjusting a value of differential pressurebetween an upstream end and a downstream end of the specimen after themeasurement cell has been closed using a lid; connecting the measurementcell to the device for producing a flow of fluid; and taking ameasurement of fluid flow using the at least one flowmeter.
 12. A methodaccording to claim 11, wherein the entire volume of the measurement cellis filled with a specimen of fibers, a mass of the specimen of fibers isequal to 5 g.
 13. A method according to claim 11, wherein the mass ofthe specimen of fibers is between 5 and 10 g.
 14. A method according toclaim 11, wherein the value of the differential pressure isapproximately 254 mmHg.
 15. A method according to claim 11, wherein themean fiber diameter of the specimen of fibers is less than 3 μm.
 16. Amethod according to claim 11, wherein the specimen of fibers areinsulation fibers of glass wool.